An artificial joint of this type may for example serve as an endoprosthesis for the human knee joint, which because of the incongruence of its osseous partners—the femoral condyle, tibial plateau and patella rear surface—requires a secondary stabilisation means. As passively acting structures, the secondary stabilisers include the capsuloligamentous apparatus and the menisci as well as the osseous portions. The muscles and tendons actively contribute to the stability. These secondary stabilisers delimit the extent of movement of the joint. The resulting degree of stability is of particular importance for the knee joint.
The extension/flexion of the joint represents the main plan of movement and is a combination of a rolling and a sliding movement of the femoral condyle over the tibial plateau. The front and rear cruciate ligament together form the central pillar of the knee joint. They stabilise the rolling and sliding movement of the knee joint and prevent the joint surfaces from sliding against one another.
Sudden uncontrolled twisting movements of the knee joint, which are not supported by the musculature, can lead to tearing of the cruciate ligaments. In most cases it is the front cruciate ligate that is affected, injuries to the rear cruciate ligament being rather rare.
In practice, a surgical cruciate ligament replacement procedure is often required in cases of this type. In this case, internal anchoring of a cruciate ligament replacement tendon is often undertaken in the joint, with an operating method involving little trauma. Through a small cut on the tibial plateau, the tendon of an adductor muscle is separated from the muscle under the skin with a special instrument.
This tendon, which is approximately 30 cm long, is then folded on top of itself four times, resulting in a replacement tendon for the cruciate ligament. This tendon must be introduced into the joint anatomically correctly in the place of the torn cruciate ligament. For this purpose, drilled channels through the tibial plateau and the thigh bone are produced with the help of aiming devices. The tendon is then drawn into the joint through the two drilled channels. There are various methods for stably fixing the tendon in the bone channel. The decision as to which method is to be used depends on individual anatomical properties of the patient, such as the length and thickness of the tendon.
A conventional joint which is configured as a knee joint prosthesis is described, for example, in DE 696 23 861 T2, in which a central rib, which is associated with the tibial joint part, engages in a cavity of the femoral joint part. In the extended position the wall of the cavity lies against the front side of the rib. There thus results a stop which is effective only in the extended position, i.e. in a single position.
A joint is described, for example, from EP 07 34 700 A 2. In this document, the joint geometry of the functional surfaces relative to one another is determined in each of the two planes by a joint chain with two joint axes, which extend through the centres of rotation of the functional surfaces with the radii of the respectively associated cross-sections, a connection which is on the femoral side, and thus on the condyle side, of the central points of the condyles corresponding to a frame, and a connection which is on the tibial side, and thus on the joint socket side, of the central points of the joint sockets corresponding to a connecting rod of a ligament chain comprising the four axes.
An artificial joint, in particular an endoprosthesis for the replacement of natural joints, consisting of at least two artificial joint parts with curved articulation surfaces, is described in DE 196 46 891 A1, a curved contact line being formed on each of the articulation surfaces. The curved contact line of one of the articulation surfaces is a portion of an elliptical cross-section of a first cylinder or cone with the cylinder radius or the cone angle. The other contact line occurs as a counter track of a second cylinder or second cone, with the cylinder radius or with the cone angle, rolling or sliding on the first cylinder or first cone. The articulation surfaces comprise control surfaces formed from a plurality of straight touch lines. These control surfaces are mutually attached to the contact lines lying opposite one another and the touch lines are in each case the connection lines between an instantaneous contact point of the contact lines and an instantaneous common point of the instantaneous pole of the respective movement systems in a reference plane or a reference sphere in the moved or unmoved system.
DE 195 21 597 A1 also relates to an artificial joint, in particular an endoprosthesis for the replacement of natural joints, consisting of at least two artificial joint parts with curved articulation surfaces, an arc-shaped contact line being formed on each of the articulation surfaces and in each case being a partial portion of a contact circle which lies in a plane and has a centre point. The articulation surfaces are arranged relative to one another as a pair, in such a way that the contact lines can roll on one another, and axes which extend perpendicular to the plane of the contact circles through the centre point thereof intersect at an intersection point. On one side, on the contact lines, control surfaces, which are formed from a plurality of straight touch lines, are formed, the touch lines lying on instantaneous connection lines of the instantaneous contact points, which occur during the rolling motion, with the instantaneous intersection points, which result from a pivoting movement of the contact lines at an angular speed around a common tangent of the contact lines through the instantaneous contact points.
EP 600 806 A1 even describes a knee prosthesis in three parts, with a femoral and a tibial implant, the femoral implant having an indentation, which circumscribes two condyle support saddles, which extend apart from one another and are connected by a trochlea, the femoral implant having a shape which results from the combination of the following features: that namely the inner and the outer condyle saddle have different radii of curvature in the sagittal plane, the bearing and sliding surfaces of the inner and outer condyle saddle have differences in width and cross-section in the frontal plane, the inner and outer condyle saddle have differences in rolling amplitude in the rear part, the trochlea delimits a surface with the cross-section of a geometric torus in the frontal and sagittal plane, said trochlea being raised on the outer side in an anatomical manner, the span of the bearing and sliding surfaces of the condyle saddle is increased in comparison to the anatomical ratios, and the condyle saddles are flattened, in order to promote the distribution of pressure.
An artificial joint which is intended to replace of human joints is known from the German patent application DE 39 08 958 A 1 and consists of two joint parts with movable functional surfaces. The ratios of curvature of the functional surfaces which have a circular cross-section are convex-convex, convex-concave, or concave-concave relative to one another, and the joint geometry is determined by a joint chain with two joint axes (dimer joint chain), which extends through the centres of rotation of the functional surfaces. In this case, the joint surfaces are formed so as to be spherical, in such a way that joint movement with five degrees of freedom is possible.